Experiment 9 Kinetics of t Bu Cl Hydrolysis Student Notes PDF

Preview

Summary

Experiment 9 student notes...

Description

! EXPERIMENT

RATES&OF&REACTION:&

9

THE!HYDROLYSIS!OF!tertiary-BUTYL!CHLORIDE! KEY&L EAR NING &CO NCEPTS KEY&LEAR EARNING NING&CO &CONCEPTS NCEPTS&& •

To!develop!an!understanding!of!the!relationship!between!experimental!data!and!the! rate!of!change!of!a!chemical!reaction.!

•

To!apply!mathematically!derived!relationships!to!experimental!data.!

•

To!understand!the!physical!significance!of!the!rate!constant!and!the!half-life! relationship!with!temperature!change.!

!

AIMS&OF&THE&EXPERIMENT&& AIMS&OF&THE&EXPERIMENT •

To!determine!the!rate!of!a!chemical!reaction.!

•

To!measure!the!kinetics!of!a!first!order!reaction!and!to!use!the!rate!law!to!determine!the! rate!constant!for!the!reaction.! To!estimate!the!activation!energy!using!the!experimentally-determined!temperature! dependence!of!the!rate!constant!of!the!reaction.! To!demonstrate!that!the!conductivity!of!a!solution!can!be!used!to!follow!the!change!in! concentration!of!reactions!that!generate!or!consume!ions.!

• •

READING READING&&

!

•

Chemistry,Human,Activity,,Chemical,Reactivity,!Mahaffy,!Bucat,!Tasker,!Kotz,!Treichel,! Weaver!and!McMurry!2nd!ed.!2015:!! The!concept!of!reaction!rate:!Section!18.2,!pages!721!–!723.! Dependence!of!rate!on!reactant!concentration:!Section!18.4,!pages!727!–!730.! Concentration-time!relationship:!integrated!rate!equations:!Section!18.5,!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! pages!732!–!736.! Half-life!of!first-order!reactions:!Section!18.5,!pages!737!–!739.! The!Arrhenius!equation:!Section!18.6,!pages!744!–!747.!

•

Online!guides!–!see!LMS!“Laboratory!Information”:! • See!CHEM!1000!Lab!Manual!2020!Techniques!and!instrumentation!!!!

1

PRELAB&QUESTIONS PRELAB&QUESTIONS&& There!is!a!ChemCAL!Prelab!module,!which!may!be!accessed!from!the!LMS!and!will!provide!some! exercises!related!to!this!experiment.!!!

& INTRODUCTION&& INTRODUCTION Tertiary-butyl!chloride!(t-BuCl)!reacts!with!water!(hydrolyse)!to!produce!tertiary-butyl!alcohol! (t-BuOH)!accordingly!to!Figure!9.1.!The!bulkiness!of!the!reaction!centre!favours!a!unimolecular! reaction!path!(i.e.!dissociation!of!Cl-!followed!by!entry!of!OH-).!

! Figure'9.1:!The!hydrolysis!of!tertiary-butyl!chloride!(t-BuCl).!

The!equation!for!the!hydrolysis!can!be!written!in!shorthand!form:! t-BuCl(aq)!+!H2O(l)!®!t-BuOH(aq)!+!H+(aq)!+!Cl-(aq)!!

,

(9.1)!

Reaction'rate:! −

The!rate!of!the!reaction!may!be!followed!by!observing!either!the!increase!in!concentration!of!H+,!Cl ! or!t-BuOH!or!the!disappearance!of!t-BuCl! !

−𝑑 𝑡-BuC𝑙

+𝑑 H+ +𝑑 𝑡-BuOH +𝑑 Cl* 𝑑𝑡 = 𝑑𝑡 = 𝑑𝑡! 𝑑𝑡 =

(9.2)!

The!reaction!(equation!9.1)!is!first!order!and!its!rate!is!accurately!described!by:! !

−𝑑 𝑡-BuCl

𝑑𝑡 = 𝑘 𝑡-BuCl !

(9.3)!

where'k'is'the'observed'rate'constant.' By!integration!this!gives:! !

[t-BuCl]t!!=![t-BuCl]0!,e−kt!

(9.4)!

ln[t-BuCl]t!=!ln[t-BuCl]0!–!kt,!

(9.5)!

or,!more!conveniently,!

!

where![t-BuCl]t!is!the!concentration!of!t-BuCl!at!any!time!t!during!the!reaction!and![t-BuCl]0!is!the! initial!concentration!of!t-BuCl.! If'a'plot'of'ln[t-BuC])t'against't'gives'a'straight'line'then'the'reaction'is'confirmed'to'be'first'order' and'the'gradient'of'the'line'will'be'−k'and'the'intercept,'at't'='0,'is'ln[t-BuCl]0.

!

2

Conductivity'and'kinetics'–'How'do'we'monitor'the'amount'of't-BuCl'still'unreacted'' at'any'time't,'[t-BuCl]t?' Hydrolysis!of!uncharged!t-BuCl!produces!t-BuOH,!H+(aq)!and!Cl−(aq).!Since,the,conductivity,of,the, neutral,molecules,is,zero,the,concentration,of,H+(aq),and,Cl-(aq),can,be,determined,by,measuring,the, conductivity,of,the,solution.!(Conductivity!is!a!measure!of!the!ability!of!a!substance!to!conduct! electricity.!Details!of!conductance!and!conductivity!measurements!can!be!found!in!Appendix!9.1!at! the!end!of!this!experiment).!

The!conductance!of!a!solution!depends!on!! § the!concentrations!of!ions! § the!charges!on!ions! § the!speed!at!which!the!ions!move! between!the!electrodes!(see!conductivity!cell!in!photo)!

'

!

Initially,!the!conductivity,!k!of!the!aqueous!solution!is!very!low!and!increases!in!proportion!to!the! concentration!of!the!H+!and!Cl-!which!are!side!products!of!the!hydrolysis!reaction.!!The'conductivity' reaches'a'maximum'when'the'reaction'is'complete.''' At'completion,'t'='¥ ,'all'the't-BuCl'present'at'the'start'of'the'experiment,'i.e.'[t-BuCl]0,'has'reacted' to'form'equal'amounts'of'H+(aq)'and'Cl-(aq).' The'conductivity'at'this'infinity'time,'k¥ ,'is'proportional'to'the'amount'of't-BuCl'present'at'the' start'of'the'reaction,'[t-BuCl]0.' The!conductivity,!kt!is!proportional!to!the!amount!of!product!ions!formed,!therefore!it!is!also! proportional!to!the!amount!of!t-BuCl!which!has!reacted,!so!we!can!write:'

k¥ ,-,kt, ,µ ,,{,[t-BuCl]0!-![t-BuCl]reacted!!}!µ!![t-BuCl]t! k¥ ,-,kt, =,Const,×![t-BuCl]t, Thus! !

ln(k¥ -, kt),=!ln(Const)!+!ln([t-BuCl]t)!

(9.6)!

To!apply!equations!9.5!and!9.6,!it!is!not!necessary!to!know!the!actual!Const!or!even!the! concentration!of!t-BuCl!at!any!time,!it!is!the!rate!of!change!of!ln(k¥!-!kt)!that!gives!the!rate!constant.! So'instead'of'plotting'ln([t-BuCl]t)'vs'time,'we'are'going'to'plot'ln(k¥ '-'kt)'vs'time'and'obtain'k,'the' rate'constant.' ' Temperature'effect'on'the'reaction'rate' It!is!generally!found!that!the!rate!constant!of!a!reaction!increases!with!increasing!temperature.!The! Arrhenius!relationship!(Chemistry,Human,Activity,,Chemical,Reactivity,!Mahaffy,,Bucat,,Tasker,,Kotz,, Treichel,,Weaver,and,McMurry,2nd,ed.,2015:,The,Arrhenius,equation:,Section,18.6,,pages,744,–,747)! describes!this!variation!as!

!

3

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,𝑘

*01

= 𝐴𝑒

-

2-

(9.7)!

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,!

or! !

𝑙𝑛 𝑘

= 𝑙𝑛𝐴 −

-

𝐸5

(9.8)!

𝑅𝑇 ,,!

where!A!is!a!constant!and,Ea!is!the!activation!energy.! At!low!temperatures,!few!molecules!possess!sufficient!energy!to!react!when!they!collide;!however,! at!higher!temperatures,!many!more!molecules!have!enough!energy!and!the!reaction!rate!is! consequently!greater.! Activation'Energy'Ea!! The!activation!energy!of!a!reaction!is!related!to!the!enthalpy!barrier!for!the!transformation!of!the! reactants!to!the!products!of!the!reaction.!Each!chemical!reaction!will!have!specific!activation!energy,! where!this!can!change!accordingly!to!the!environment,!e.g.!the!polarity!of!the!solvent.!! Rearrangement!of!the!Arrhenius!equation!(equation!9.8)!shows!that!the!temperature!dependence! of!the!reaction!rate!constant!(k)!can!be!used!to!deduce!the!activation!energy.!Where!a!plot!of!ln(k(T))! versus!1/T!should!give!a!straight!line!with!gradient!equal!to!–(Ea/R).! !

𝑙𝑛 𝑘8 = 𝑙𝑛 A −

E;

1 R ∙ T ,,,,!i.e. gradient!= −

𝐸5

𝑅

!

(9.9)!

[Note:!use!R!=!8.314!Jmol-1K-1!for!gas!constant!and!the!temperatures!are!in!Kelvin].! Alternatively,!if!the!rate!constant!is!known!at!more!than!one,!say!two!temperatures,!T1!and!T2,!then! the!activation!energy!can!be!obtained!from!the!relationship,! !

,,,,,𝑙𝑛

𝑘

-A

𝑘

,= −

𝐸5

-B

1 1 𝑅 ∙ 𝑇C − 𝑇D !

(9.10)!

This!relationship!is!commonly!expressed!as,! !

𝑙𝑛 𝑘

-A

− 𝑙𝑛 𝑘

-B

=,−

𝐸5

1 1 𝑅 ∙ 𝑇C − 𝑇D

!

(9.11)!

Measurement'of'Conductivity' Direct'Reading'Conductivity'Meters:'The!conductivity!is!measured!at!regular!intervals!during!the! reaction!and!should!increase!with!time!until!all!the!t-BuCl!has!been!converted!to!t-BuOH.!The! conductivity,!in!units!of!micro-Siemens!per!centimetre!(µS!cm-1),!is!displayed!directly!and!the! conductivity!meters!have!been!pre-calibrated.! Conductivity'Cell:'The!conductivity!cell!used!in!this!exercise!consists!of!a!probe!which!sits!inside!a! glass!vessel.!The!probe!is!fitted!with!a!pair!of!specially!prepared!platinum!electrodes,!through!which! electrical!contact!with!the!solution!is!made.!The!probe!may!be!removed!for!the!purposes!of!rinsing! and!filling!the!conductivity!cell.!

!

4

EXPERIMENTAL&PROCEDURE EXPERIMENTAL&PROCEDURE&&AND&FILM AND&FILM&& There'is'a'complete'report'template'for'students'to'complete'and'submit.''All'your'results,' answers'and'report'format'is'in'the'template'-'you'do'not'need'to'copy'or'write'any' supplementary'report.' Open'the'report'template'document'on'your'computer'or'device'so'you'can'enter'data'from'the' film.''Alternatively,'you'may'prefer'to'print'it'and'have'it'ready'for'entering'data.' Watch'the'film'and'enter'data'into'your'report'where'directed.''You'can'follow'along'the' procedure'in'your'notes'or'simply'watch'the'film'and'read'over'your'notes'separately.' You'can'complete'any'parts'of'your'report'before'your'Lab'session'that'you'feel'confident'to'do.'' Practice'the'calculations.''Prepare'well'and'have'questions'ready'to'ask'your'demonstrator.' It*is*highly*recommended*that*you*aim*to*complete*all*your*report*during*your*lab*session'while' you'are'able'to'question'your'demonstrator.''This'is'what'would'be'expected'in'a'normal'face'to' face'laboratory'class.' Part'A:' Reaction'rate'measurements' The!rate!of!the!hydrolysis!reaction!of!t-BuCl!depends!on!the!polarity!of!the!solvent!and!is! inconveniently!fast!in!pure!water!(the!half-life!of!the!reaction!is!about!20!seconds).!The!reaction!rate! is!greatly!slowed!down!in!the!presence!of!ethanol!and!we!shall!therefore!study!the!reaction!in!a! mixture!of!water,!61%!and!ethanol,!39%!(by!volume).!It,is,important,that,the,composition,of,the, solution,used,for,measurements,does,not,change,when,transferring,solutions,between,containers,or, to,the,conductivity,cell.! The!rate!of!the!hydrolysis!will!be!measured!at!25!°C,!30!°C,!35!°C!or!38!°C,!using!a!temperature! controlled!water!bath.! The!set!of!solutions!provided!consists!of:!! •

10-2!M!solution!of!t-BuCl!in!pure!ethanol!at!room!temperature.!

•

Ethanol-water!solution!(39%!by!volume!of!ethanol)!in!water!bath!adjusted!to!the!required! temperature.!

NOTE:,The,conductivity,cells,are,made,of,glass,and,must,be,handled,with, care.' No,air,bubbles,should,be,trapped,in,the,cells,when,making,conductivity, measurements.,

EXPERIMENTAL&PROCEDURE&& EXPERIMENTAL&PROCEDURE

Preparing! Solutions!

Reaction'mixture'for'your'kinetic'run' Note:,It,is,important,to,be,able,to,obtain,reliable,kinetic,results,within,a,¼,half-life.,At,higher, temperatures,,the,half-life,of,the,reaction,is,about,8,mins,so,the,first,reading,is,made,at,2, minutes,after,mixing,t-BuCl,with,water.,It,is,important,for,students,to,work,effectively,together.,

!

5

The,LabByte,shows,the,preparation,of,the,solution,and,filling,of,the,conductivity,cell,and,the, flowchart,,Figure,9.2,!on,the,following,page,will,help,your,planning,of,the,experiment.,

!

i.

Collect!the!cap!and!numbered,!DRY!150!mL!plastic!bottle!from!pin-dryer.!!Make*sure*you* remember*your*bottle*number.*

ii.

Dispense!10!mL!of!the!t-BuCl!stock!solution,!via!the!zippette,!into!the!plastic!bottle!and!cap.!

iii.

Equilibrate!the!solution!in!your!water!bath!for!5!minutes.!

iv.

Using!a!measuring!cylinder,!add!90!mL!of!the!ethanol-water!mixture!equilibrated!at!the! temperature!of!your!measurement!to!the!t-BuCl!solution!and!start'the'stop-clock' immediately.![Note:*This*is*taken*as*the*zero*Time*(t*=*0*seconds)*at*the*temperature*(T)*of* your*hydrolysis*run.]'

v.

Shake!well!to!mix.!

vi.

While!your!partner!is!preparing!the!solution!(Parts!iv!and!v),!empty!the!conductivity! assembly.!!

vii.

Then!rinse!the!conductivity!assembly!twice!with!the!reaction!mixture!(from!Part!v).! Approximately!5!mL!is!sufficient!for!each!rinse.!

viii.

Fill!the!cell!with!the!reaction!mixture!well!above!the!electrode!and!place!it!in! the!water!bath!at!pre-set!temperature.!

ix.

Cap'the'plastic'bottle'containing'the'remaining'reaction'mixture'(should'be' about'50'mL)'and'place'in'the'water'bath'set'at'45'°C'(this'will'be'your't¥' solution).'

!

k¥!Measurement!

x.

Make!your!first!measurement!of!the!conductivity!at!time!=!2!minutes!(the! conductivity!cell!should!have!been!equilibrating!for!about!1!minute).!!Record*your*results*in* a*pre-prepared*table*in*your*laboratory*notebook.,

xi.

Make!readings!of!conductivity,! 25,or,30,°C:! at!two-minute!intervals!for!the!first!20!minutes,!then!at!5!minute!intervals! for!a!further!20!minutes.!! , 35,°C:! at!two-minute!intervals!for!the!30!minutes!of!the!kinetic!scan.!! ! 38,°C:, at!one-minute!intervals!for!the!first!16!minutes,!then!at!2!minute!intervals! for!a!further!14!minutes.!! ! Overall!the!readings!are!taken!over!40!minutes!(25!or!30!°C)!or!30!minutes!(35!or!38!°C)!of! time!period.! !

6

Time/mins 0

Transfer, 10, mL, of, t-BuCl, to,150,mL,plastic,bottle Measure, 90, mL, of,water:Ethanol Add,water:ethanol,solution,to,t-BuCl, Mix

Transfer,remaining,solution,(>,50,mL),to,

Rinse,conductivity,cell,and,fill,to,mark,(5-6,cm) Equilibrate,temp 2

22 32 42

45,∞ C,water,bath,(t¥,solution)

Record,conductivity C Temp,=,25,or,30, ∞

Temp,=,35,∞ C

Temp,=,38,∞ C

measure,conductivity, every,2,mins,for,20,mins

measure, conductivity,

THEN

every,2,mins, for,30,mins

meas.,conduct., every,1,min,for, 16,mins,,then, every,2,mins,for, 14,mins

measure,every,5,mins,for, 20,mins

End,of,kinetic,run, C, 42,mins,for,25,or,30,∞

transfer,t¥,solution,from,the,45,∞ C, water,bath,to,the,bath,used,for,your, measurements

32,mins,for,35,or,38,∞ C

50

Rinse, conductivity, cell, with, the, , t¥ solution,and,and,fill,to,mark,(5-6,cm) Equilibrate,for,5,minutes Record, the, conductivity, of, the, t¥, solution

Figure'9.2:'The!flow!chart!for!the!hydrolysis!run.!

!

3.! Readings'of'conductivity'of'your't¥'solution'( k¥)' Description,of,the,measurement,procedure,and,outline,of,the,importance,of,conductivity,at,t ¥, (k¥ ),can,be,found,in,the,LabByte,,a,discussion,of,the,impact,on,the,analysis,is,given,in,Appendix, 9.2,at,the,end,of,this,experiment,on,page,11., i.

Once!your!t ¥,solution!(the!remaining!reaction!solution)!has!equilibrated!in!the!45!°C!water! bath!for!30!minutes!move!the!plastic!bottle!from!the!45!°C!water!bath !to!the!water!bath! used!for!your,kinetic!measurements!!!!!!!!!!!!!!!(25!or!30!or!35!or!38!°C)!and!equilibrate!for!a! further!10!minutes.!

ii.

Empty!the!conductivity!assembly!upon!the!completion!of!the!hydrolysis!run.!Rinse!twice! with!the!t¥,solution.,Remember,to,gently,open,the,plastic,bottle,as,pressure,can,build,up, inside,during,the,reaction,time.!

iii.! Rinse!the!conductivity!cell!(twice,!but!less!than!5!mL!for!each!rinse)!and!fill!the!conductivity! cell!with!your!remaining!t¥,solution.! iv.! Once!the!temperature!in!the!conductivity!cell!has!stabilised,!measure!and!record!the! conductivity!(k¥).!This!usually!takes!no!more!than!5!minutes.! '

!

'

7

Submitting'your'report' When'you'have'completed'your'report'in'the'document'provided'on'the'LMS,'proceed'to'create' ONE'pdf'document.' If'you'have'entered'your'answers'and'calculations'directly'into'the'electronic'version'of'the' report,'simply'save'the'pdf.' If'you'have'handwritten'part'or'all'of'your'report,'either:' i.'scan'the'pages'using'a'mobile'telephone'app'(recommended'Scannable'(for'iOS)'or'Genius'Scan' (Android))'and'compile'into'ONE'pdf,'or' ii.'take'photographs'of'the'pages'and'merge'and'compress'into'ONE'pdf.' Submit'your'report'to'the'appropriate'assignment'tab'(Experiment'#,'Session'day/time/group'no)' under'ONLINE'PRACTICAL'ASSIGNMENT'in'Assignments'on'the'LMS.' ' !

!

&

8

APPENDIX&9.1&& APPENDIX&9.1 Conductance'and'conductivity' A!material!conducts!electricity!to!an!extent!described!by!its!conductance,!G,!which!is!the!reciprocal! of!its!resistance,!R.!If!a!voltage!V!is!applied!to!a!piece!of!material!and!a!current,!I,!flows!(Figure!9.3A),! then!Ohm’s!law!states:!

𝐼 1 =𝐺= ! 𝑅 𝑉 G!will!depend!on!the!dimensions!of!the!material,!particularly!with!its!length!l,!and!its!cross-sectional! area!A!(Figure!9.3B);!in!fact,!

𝐺=

𝐴 𝜅! 𝑙

The!proportionality!constant!k!is!known!as!the!conductivity!of!the!material.!It!is!the!conductance!of! a!block,!1!m!long!and!1!m2!in!cross-sectional!area.! k!enables!us!to!compare!the!intrinsic!conducting!power!of!one!substance!with!another,!free!of!the! influence!of!sample!size!and!shape.!

I

l A

V A

B

!

Figure'9.3:!Conductance!

Properties'of'k'for'electrolyte'solutions' Strong!electrolytes,!e.g.!salts!such!as!NaCl,!when!dissolved!in!water!completely!dissociate!into!ions! and!give!highly!conducting!solutions.!The!conductivity!of!such!a!solution!depends!on!the! concentration!of!the!ions,!the!number!of!charges!on!each!ion,!and!the!speed!at!which!they!move! (mobility)!when!an!electric!field!is!applied.! The!variation!of!conductivity!k!with!concentration,!C,!for!several!electrolytes!is!shown!in!Figure!9.4.! At!very!high!concentrations!the!conductivity!becomes!non-linear,!mostly!a!result!of!ion!pairing.!At! lower!concentrations,!the!graphs!are!nearly!linear!but!of!different!slope!for!different!electrolytes.!To! compare!the!conducting!power!of!different!electrolytes!we!need!to!factor!out!the!effect!of! concentration!by!defining!the!molar!conductivity!or!conductivity!per!unit!concentration,! 𝜅 Λ= ! 𝐶 Note!that,!as!curvature!in!Figure!9.4!implies,!L!is!only!approximately!a!constant!for!any!electrolyte,! and!tends!to!fall!off!at!higher!concentration,!i.e.!the!conductivity!per!unit!concentration!of!ions!is! less!in!higher!concentration!solutions!where!the!ions!are!moving!less!freely.!The!limiting!value!of!L! at!low!concentration!is!often!listed!in!tables!as!L˚!‘the!molar!conductivity!at!infinite,dilution’!

!

9

Note!also!that!the!k!values!for!the!three! chlorides!are!all!different;!HCl!>!KCl!>!NaCl.!The! anion!is!common!in!all!these!solutions!so!the! differences!indicate!that!the!cations!have! different!mobilities,!i.e.!H+!>!K+!>!Na+.!(Similar! observations!would!be!made!if!you!studied! solutions!of!NaNO3,!NaCl,!and!NaBr;!common! cation,!various!anions).! Observations!such!as!this!led!to!the!idea!of!the! independent!migration!of!ions.! The!molar!conductivity!of!an!electrolyte!can!be! expressed!as!the!sum!of!contributions!of!its! individual!ions;! !e.g.!for!NaCl:!λ(NaCl)!=!λ+!+!λ-! where!λ+!and!λ-!are!molar!conductivities!of!the! sodium!cation!and!chloride!anion,!respectively.!! For!a!general!electrolyte!AxBy:!

λ(AxBy)=!x·λ+!+!y·λ-! ! Figure'9.4:!Conductivity!of!various!electrolyte! solutions!(top!–!experimental!for!concentrated! solutions,!bottom!–!schematic).'

e.g.!for!aluminum!sulfate,!Al2(SO4)3:!

L(Al2(SO4)3) =!2λ(AI3+)!+!3λ(SO42−)!

T...